Electromechanical conversion devices such as an ultrasonic conversion device are used widely for transducing an electric signal to an ultrasonic wave and vice versa, and are useful as sensing probes for medical imaging or nondestructive inspection.
One type of such an electromechanical conversion device is a capacitive micro-machined ultrasound conversion device (CMUT). An example of a CMUT is constituted of an element substrate containing plural elements having a lower electrode, a membrane as a vibrator supported by a supporter placed on this substrate, and an upper electrode, and a circuit substrate connected electrically to the element substrate. This CMUT has a cavity as a gap between the substrate and the membrane. The CMUT emits an ultrasonic wave by vibrating the membrane by application of a voltage between the upper electrode and the lower electrode, or detects an ultrasonic wave by a change of the capacitance caused by vibration of the membrane on reception of an ultrasonic wave.
The element substrate can be produced by surface micro-machining or bulk micromachining. In an example of the surface micromachining method, a silicon nitride film is formed as the membrane on a sacrificial layer on a substrate; an etching hole is bored therein; a cavity is formed by etching the sacrificial layer through the etching hole; and finally the etching hole is filled with silicon nitride to form a vacuum cavity. In an example of the bulk machining method, a cavity structure is formed on a silicon substrate, and thereto an SOI substrate (silicon-on-insulator substrate) is bonded. This method gives improved mechanical properties owing to mono-crystalline silicon used as the membrane.
A document, Sensors and Actuators A 138 (2007) 221-229 (hereinafter referred to as “Non-Patent Document 1”) discloses a method in which membranes and cavities are formed two-dimensionally on a silicon substrate, and the silicon substrate itself is connected, as a lower electrode and wiring, to a circuit substrate. This method is described below with reference to FIG. 12. Element substrate 1007 is constituted of a plurality of elements. The element as a unit emits or receives an ultrasonic wave. One element 1008 is constituted of upper electrode 1000, membrane 1001, cavities 1002, support 1009, and lower electrode 1003. Grooves 1004 are provided to surround the respective lower electrodes to insulate the elements from each other to isolate the lower electrodes for the respective elements. Element substrate 1007 is connected to ASIC substrate 1006 (“application specific integrated circuit” substrate) through bumps 1005 as the electric contacts. The silicon substrate can be worked to form the lower electrodes and wiring lines at a high temperature without damage. Either surface micro-machining or bulk micro-machining can be employed for machining thereof.